1. Field of the Invention
This invention relates to a semiconductor amplifier. More specifically, it relates to a wide-band DC amplifier having a moderate power output and characterized by faithful reproduction of its input signals.
2. Prior Art
In many DC amplifier applications it is desirable to provide a moderate output power, e.g. 2 or 3 watts, together with a low output impedance. The latter characteristic is usually accomplished by means of negative feedback which, however, adversely affects the high-frequency or transient response of the amplifier. Specifically, with negative feedback the output of the amplifier is characterized by excessive delay or overshoot in response to sudden changes in the input voltage.
One might instead resort to an essentially open-loop amplifier configuration with an emitter-follower output stage which provides the desired low-impedance output characteristic. However, a conventional, i.e. push-pull, output stage is subject to thermal runaway and consequent burnout of one or both of the output transistors unless the amplifier includes provisions for preventing this from happening. For example, emitter resistors might be connected in series with the emitters of the output transistors, but this would increase the output impedance.
One solution to the thermal runaway problem has been to use a monolithic amplifier construction in which all of the transistors are formed on the same semiconductor chip. All of the transistors thus have essentially the same temperature. Accordingly, an increase in the temperature of one of the output transistors, which would otherwise increase the current through that transistor, will, by virtue of the temperature increase in other transistors in the circuit, cause an offsetting reduction in the bias voltage applied to the output transistors, thereby preventing an excessive increase in quiescent current and, consequently, eliminating runaway.
However, the latter circuit is characterized by temperature-dependent offsets in the output voltage. Moreover, the output current capability is not as high as desirable for a power amplifier because, in a complementary monolithic amplifier, transistors of one type, npn or more likely pnp, have inherently high collector resistances or other deficiencies. Therefore, inverse feedback is still required for operation of the amplifier.